scsi: replace the fmode_t argument to ->sg_io_fn with a simple bool

Instead of passing a fmode_t and only checking it for FMODE_WRITE, pass
a bool open_for_write to prepare for callers that won't have the fmode_t.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230608110258.189493-21-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>